It is becoming apparent that electronic systems that combine new energy storage technologies with electric drive technology will be ever more widely used in future both in stationary applications, such as for example wind energy plants or solar energy systems, also in vehicles, such as hybrid vehicles or electric vehicles. Nowadays, electric machines are being driven by way of example by means of a combination of battery cells in strings, wherein a downstream AC converter provides the supply voltage to the electric machine.
By way of example, a DC voltage that is provided by a DC voltage intermediate circuit is converted into a three phase AC voltage as alternating current is supplied to the supply connectors of a three phase electric machine or of a three phase supply network by way of a converter in the form of an AC pulse converter. The DC voltage intermediate circuit is for its part supplied by a string of battery cells. In order to be able to fulfill the pre-existing requirements relating to power and energy for a specific application, several battery cells are connected in series.
In the case of methods of this type, it is desirable to connect individual battery cells selectively into the supply circuit in order either to gain the advantages of the system in the downstream electronic power output stage or to purposefully remove from the supply circuit individual battery cells that as a result of aging or the tasks involved over time no longer provide the required power. Battery cells age as a result of the amount of their energy output and/or power output, the operating temperature and other parameters. Fluctuations in performance as a result of manufacturing tolerances or fluctuating environmental conditions of the individual battery cells cause some battery cells to age more quickly than other battery cells. In the case of a plurality of battery cells that are connected, the differing aging conditions can cause a load imbalance between the individual battery cells, as a result of which the weakest battery cells determine the overall performance of the supply circuit. The method known from the prior art of balancing said battery cells on the basis of the cell voltage does not differentiate between whether a load imbalance has been caused by extremely high power consumptions over short periods of time or by low power consumptions over longer periods of time.